Spin-orbit torque devices

ABSTRACT

An example article includes a composite free layer and a conductive channel. The composite free layer includes a high-anisotropy ferromagnetic layer, a non-magnetic transition metal layer adjacent to the high-anisotropy ferromagnetic layer, and an ultra-low damping magnetic insulator. The non-magnetic transition metal layer is between the ultra-low damping magnetic insulator and the high-anisotropy ferromagnetic layer. An example spin-orbit torque (SOT) stack may include the example article. Techniques for forming and switching example articles and SOT stacks are described.

This application claims the benefit of priority to U.S. Provisional Patent Application No. 62/661,407, titled, “SPIN-ORBIT TORQUE DEVICES,” filed Apr. 23, 2018, the entire content of which is incorporated herein by reference.

GOVERNMENT CLAUSE

This invention was made with government support under Grant No. HR0011-13-3-0002 awarded by the Department of Defense/Defense Advanced Research Projects Agency (DARPA). The government has certain rights in the invention.

TECHNICAL FIELD

The present disclosure relates to spin-orbit torque devices.

BACKGROUND

Spin-transfer torque RAM (STTRAM) is a non-volatile, zero static power alternative to Silicon-based memories. STTRAM is a candidate for next generation memory as complementary metal-oxide-semiconductor (CMOS) technology begins to hit physical limits, including high leakage currents, heating issues, and the like that beget intractable refresh rates as dynamic random-access memory (DRAM) scales to higher densities. However, STTRAM comes with its own design challenges.

SUMMARY

In general, the present disclosure is directed to spin-orbit torque (SOT) devices, and techniques for making and switching SOT devices or articles including SOT structures.

In some examples, the disclosure describes an article including a composite free layer. The composite free layer includes a high-anisotropy ferromagnetic layer, a non-magnetic transition metal layer adjacent to the high-anisotropy ferromagnetic layer, and an ultra-low damping magnetic insulator. The non-magnetic transition metal layer is between the ultra-low damping magnetic insulator and the high-anisotropy ferromagnetic layer. The example article includes a conductive channel including a heavy metal region adjacent the composite free layer. The ultra-low damping magnetic insulator is between the non-magnetic transition metal layer and the conductive channel.

In some examples, the disclosure describes an example technique including depositing a non-magnetic transition metal layer on an ultra-low damping magnetic insulator. The ultra-low damping magnetic insulator is on a conductive channel comprising a heavy metal region. The example technique includes depositing a high-anisotropy ferromagnetic layer on the non-magnetic transition metal layer.

In some examples, the disclosure describes an example technique including inducing spin orbit torque by passing a current through a heavy metal region of a conductive channel adjacent a composite free layer. The composite free layer includes a high-anisotropy ferromagnetic layer, a non-magnetic transition metal layer adjacent to the high-anisotropy ferromagnetic layer, and an ultra-low damping magnetic insulator. The example technique includes switching a magnetization of the ultra-low damping magnetic insulator in response to the spin orbit torque. The example technique includes switching a perpendicular magnetization in the high-anisotropy ferromagnetic layer in response to the switching of the magnetization of the ultra-low damping magnetic insulator. The high-anisotropy ferromagnetic layer is exchange-coupled to the ultra-low damping magnetic insulator.

The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques described in this disclosure will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a conceptual diagram illustrating a side view of an example article including a composite free layer including a high-anisotropy ferromagnetic layer, a non-magnetic transition metal layer and an ultra-low damping magnetic insulator.

FIG. 1B is a conceptual diagram illustrating a top view of the article of FIG. 1A.

FIG. 2 is a flow diagram illustrating an example technique for forming an example article including a composite free layer.

FIG. 3 is a flow diagram illustrating an example technique for switching an example article including a composite free layer.

FIG. 4A is a chart illustrating relationships between write energy, exchange coupling, and magnetic insulator thickness for an example device having a diameter of 13.2 nm.

FIG. 4B is a chart illustrating relationships between write energy, exchange coupling, and magnetic insulator thickness for an example device having a diameter of 9.4 nm.

FIG. 4C is a chart illustrating relationships between write energy, exchange coupling, and magnetic insulator thickness for an example device having a diameter of 6.6 nm.

FIG. 5A is a chart illustrating relationships between write energy and diameters of example and comparative memory structures.

FIG. 5B is a chart illustrating relationships between write energy and diameters of example and comparative memory structures.

FIG. 6A is a chart illustrating magnetization reversal of an ultra-low damping magnetic insulator in an example antiferromagnetically coupled composite structure.

FIG. 6B is a chart illustrating magnetization reversal of a high-anisotropy ferromagnetic layer in the antiferromagnetically coupled composite structure of FIG. 6A.

FIG. 7 is a chart illustrating magnetization switching of an ultra-low damping magnetic insulator and a high-anisotropy ferromagnetic layer in an example antiferromagnetically coupled composite structure.

FIG. 8A is a chart illustrating relationships between write energy, exchange coupling, and magnetic insulator thickness for an example device having a diameter of 8.3 nm and including an FePt high-anisotropy ferromagnetic layer.

FIG. 8B is a chart illustrating relationships between write energy, exchange coupling, and magnetic insulator thickness for an example device having a diameter of 15.6 nm and including an FePd high-anisotropy ferromagnetic layer.

FIG. 9 is a chart illustrating bit error rates for macrospin and micromagnetic simulations for an example composite structure including an yttrium iron garnet (YIG) ultra-low damping magnetic insulator and an FePd high-anisotropy ferromagnetic layer.

DETAILED DESCRIPTION

In some examples, the disclosure describes spin-orbit torque (SOT) devices, and techniques for making and switching SOT devices or articles including SOT structures. An example SOT device may include a memory structure, for example, a SOT random-access memory (SOTRAM) cell. In some examples, articles according to the disclosure may be used in ultra-high density-memory incorporating highly-efficient (for example, low power consuming), exchange-coupled SOTRAM cells. In some examples, the power consumption of some SOT devices described herein may be of factor of about 70 or so below a power consumption associated with DRAM.

In some examples, the disclosure describes an article including a composite free layer. The composite free layer includes a high-anisotropy ferromagnetic layer, a non-magnetic transition metal layer adjacent to the high-anisotropy ferromagnetic layer, and an ultra-low damping magnetic insulator. The non-magnetic transition metal layer is between the ultra-low damping magnetic insulator and the high-anisotropy ferromagnetic layer. The example article includes a conductive channel including a heavy metal region adjacent the composite free layer. The ultra-low damping magnetic insulator is between the non-magnetic transition metal layer and the conductive channel.

In some examples, the disclosure describes a technique including depositing a non-magnetic transition metal layer on an ultra-low damping magnetic insulator. The ultra-low damping magnetic insulator is on a conductive channel comprising a heavy metal region. The example technique includes depositing a high-anisotropy ferromagnetic layer on the non-magnetic transition metal layer.

In some examples, the disclosure describes a technique including inducing spin orbit torque by passing a current through a heavy metal region of a conductive channel adjacent a composite free layer. The composite free layer includes a high-anisotropy ferromagnetic layer, a non-magnetic transition metal layer adjacent to the high-anisotropy ferromagnetic layer, and an ultra-low damping magnetic insulator. The example technique includes switching a magnetization of the ultra-low damping magnetic insulator in response to the spin orbit torque. The example technique includes switching a perpendicular magnetization in the high-anisotropy ferromagnetic layer in response to the switching of the magnetization of the ultra-low damping magnetic insulator. The high-anisotropy ferromagnetic layer is exchange-coupled to the ultra-low damping magnetic insulator.

By exploiting typically unrealized benefits of spin-orbit torque (SOT), in particular, its compatibility with low-damping insulators and the energy efficiencies associated with exchange coupling of magnetically hard and soft composite structures, a write energy of 10 aJ/bit may be achieved for a 10 nm cell. Furthermore, high magnetocrystalline anisotropy (K_(u)) materials such as L1₀-FePt may be employed not only to facilitate achievement of ultra-high-density memory but to allow for reduction of heavy metal layer volume and a reduction in write energy not seen in previous CoFeB-based cells. In some examples, this energy may be within a factor 40 of the theoretical limit of 60 k_(B)T, and may also represents a 10³ improvement in energy consumption compared to state-of-the-art double data rate fourth-generation (DDR4) DRAM cells and a 10⁵ improvement in energy consumption when DRAM refresh energies are included.

In contrast with SOTRAM, STTRAM may suffer from rapid tunnel barrier degradation and an inefficient use of electron spin-both due to its current-perpendicular-to-plane (CPP) geometry. Furthermore, the insulating barrier resistivity substantially increases write energy. The barrier breakdown field generally limits switching to 1 GHz for thermally stable devices.

Spin-orbit torque RAM (SOTRAM) devices utilize spin-orbit interaction at the interface of heavy metal (HM) and ferromagnetic (FM) layers via mechanisms such as the Rashba effect and spin hall effect (SHE). This method of generating torque may be more efficient than STTRAM since the electrons travel parallel (instead of perpendicular) to the interface, enabling each electron to undergo multiple spin-flip scatterings and exceeding a single quanta of spin (ℏ/2). This inefficiency in STTRAM requires larger critical current, which flows through the tunnel barrier and accelerates its deterioration. The current-in-plane (CIP) geometry of SOTRAM allows the use of low-damping magnetic insulators (MI) in place of the FM, reducing critical current further. This benefit may be leveraged in the construction of example SOTRAM cells. Compared to STTRAM, the impedance is determined by the HM and is much lower than that of MgO. Therefore, SOT-based articles and systems may be inherently low-impedance and can operate at several mV.

FIG. 1A is a conceptual diagram illustrating a side view of an example article 10 including a composite free layer 11 including a high-anisotropy ferromagnetic layer 12, a non-magnetic transition metal layer 14, and an ultra-low damping magnetic insulator 16. FIG. 1B is a conceptual diagram illustrating a top view of article 10 of FIG. 1A. One or more layers of composite free layer 11 have switchable free magnetic fields.

Ultra-low damping magnetic insulator 16 (also referred to as “ULD”) may be a relatively easily switchable soft magnetic layer, and switching of ultra-low damping magnetic insulator 16 may promote or cause switching of high-anisotropy ferromagnetic layer 12 (also referred to as “FM”), for example, via magnetic coupling modulated by non-magnetic transition metal layer 14. In some examples, ultra-low damping magnetic insulator 16 may be antiferromagnetically coupled to high-anisotropy ferromagnetic layer 12. Thus, high-anisotropy ferromagnetic layer 12 and ultra-low damping magnetic insulator 16 may constitute a synthetic antiferromagnet that is substantially stray-field free.

In some examples, article 10 may include an ultra-high density memory cell. Article 10 may include an FM material with relatively high magnetocrystalline anisotropy K_(u) (for example, on the order of 10-100 Merg/cc) to promote high thermal stability (Δ). For example, L1₀-ordered FePt may be employed due to its high magnetocrystalline anisotropy (K_(FePt)=70 Merg/cc) and moderate Gilbert damping (α_(Fept)=0.02). As another example, L1₀ FePd may be employed. Generally, a thermal stability of 40-60 k_(B)T achieves data retention on the order of 5-10 years, where k_(B) is the Boltzmann constant and T is the absolute room temperature. A high K_(u) material like FePt or FePd may reduce the requisite device diameter and, thus, the cross-sectional area of the HM layer, which may in turn reduce switching current and write energy.

For example, high-anisotropy ferromagnetic layer 12 may include a magnetic material such as L1₀ FePt or L1₀ FePd. In some examples, high-anisotropy ferromagnetic layer 12 includes L1₀ FePt. In some examples, high-anisotropy ferromagnetic layer 12 consists of or consists essentially of L1₀ FePt. In some examples, high-anisotropy ferromagnetic layer 12 includes L1₀ FePd. In some examples, high-anisotropy ferromagnetic layer 12 consists of or consists essentially of L1₀ FePd. High-anisotropy ferromagnetic layer 12 may have a magnetic anisotropy in a range from about 1×10⁶ ergs/cc to about 4×10⁷ ergs/cc, such as a magnetic anisotropy in a range from about 1×10⁶ ergs/cc to 2.5×10⁷ ergs/cc.

High-anisotropy ferromagnetic layer 12 may have any suitable thickness, measured in a direction normal to a major surface defined by article 10 or conductive channel 18. For example, high-anisotropy ferromagnetic layer 12 may have a thickness in a range from about 3 Angstroms (Å) to about 10 nanometers (nm), such as a thickness in a range from about 3 Å to about 5 nm. In some examples, it may be difficult to uniformly deposit high-anisotropy ferromagnetic layer 12 layer with a thickness of less than 3 Å. In some examples, it may be difficult to deposit high-anisotropy ferromagnetic layer 12 layer with a thickness of greater than about 5 nm, or greater than about 10 nm, for example, using techniques such as lithography.

Non-magnetic transition metal layer 14 is between high-anisotropy ferromagnetic layer 12 and ultra-low damping magnetic insulator 16 and may promote exchange coupling of high-anisotropy ferromagnetic layer 12 and ultra-low damping magnetic insulator 16, for example, by modulating antiferromagnetic coupling. Non-magnetic transition metal layer 14 includes at least one non-magnetic transition metal. For example, non-magnetic transition metal layer 14 may include a metal or alloy including at least one non-magnetic metal belonging to groups 3d, 4d, or 5d of the elemental periodic table. In some examples, non-magnetic transition metal layer includes rhodium (Rh). In some examples, non-magnetic transition metal layer essentially consists of rhodium. Using rhodium may provide, in some examples, peak exchange coupling (J_(ex)) values of 34 erg/cm².

Non-magnetic transition metal layer 14 may define any suitable thickness. In some examples, non-magnetic transition metal layer 14 has a thickness of less than about 15 Å. In some examples, a thickness of less than 15 Å may promote antiferromagnetic exchange coupling, and a thickness of greater than 15 Å may weaken antiferromagnetic exchange coupling. In some examples, non-magnetic transition metal layer 14 has a thickness of more than about 1 Å, or more than about 3 Å, or more than about 5 Å, and/or less than about 15 Å, or less than about 10 Å.

Article 10 also includes a conductive channel 18 including a heavy metal region 20 adjacent composite free layer 11. Ultra-low damping magnetic insulator 16 may be between non-magnetic transition metal layer 14 and conductive channel 18, as shown in FIG. 1A. Ultra-low damping magnetic insulator 16 may include any suitable magnetically insulating material that is ultra-low damping, for example, having a damping constant α on the order of 10⁻⁵. Ultra-low damping associated with a on the order of 10⁻⁵ or lower may reduce the energy required to switch magnetization of composite free layer 11 or a layer of composite free layer 11. In some examples, ultra-low damping magnetic insulator 16 includes yttrium iron garnet (YIG) or barium ferrite.

Example SOTRAM structures may have a current-in-plane (CIP) geometry, and as a consequence, current may not flow through YIG. Thus, YIG may be used as an ultra-low damped soft layer, i.e., ultra-low damping magnetic insulator 16. The magnetocrystalline anisotropy of YIG (K_(YIG)) is ˜10 kerg/cc. In some examples, ultra-low damping magnetic insulator 16 includes YIG having a damping constant α_(YIG), of about 5×10⁻⁵). In some examples, ultra-low damping magnetic insulator 16 consists of or consists essentially of YIG. Ultra-low damping magnetic insulator 16 may define any suitable thickness. For example, ultra-low damping magnetic insulator 16 may define a thickness of at least about 10 nm, and less than about 1 μm, or less than about 0.1 μm (100 nm).

Conductive channel 18 may include a conductive region 26 adjacent or surrounding heavy metal region 20. Conductive region 26 may include any suitable conducting material, for example, an electrically conductive metal or an alloy. In some examples, conductive region 26 includes copper (Cu) or aluminum (Al). In some examples, conductive region 26 consists essentially of Cu.

Heavy metal region 20 (also known as “HM”) of conductive channel may include any suitable heavy metal. For example, heavy metal region 20 may include one or more of a metal or an alloy including platinum, palladium, or tungsten. In some examples, heavy metal region 20 includes β-tungsten. In some examples, heavy metal region 20 consists of or consists essentially of β-tungsten. β-tungsten may provide a relatively high SOT capability, for example, by having a relatively higher resistivity, while also scattering electrons. The thickness of heavy metal region 20 may be substantially the same as a thickness of conductive channel 18, in a direction normal to a major surface defined by conductive channel 18. In some examples, heavy metal region 20 may have a thickness different from a thickness of conductive channel 18.

Composite free layer 11 and conductive channel 18 may be part of an SOT cell, or an SOT structure, for example, an SOT memory cell. Thus, in some examples, article 10 may include an SOT RAM cell. In some examples, article 10 may include additional layers adjacent composite free layer 11 to “read” or “write” a memory bit from or to article 10, for example, by detecting a magnetic state, or by switching a magnetic state of article 10, or of composite free layer 11, or of a layer of composite free layer 11. In some examples, article 10 further includes a reference layer 22 and a barrier layer 24 adjacent composite free layer 11 and opposing conductive channel 18. For example, reference layer 22 and barrier layer 24 may be adjacent high-anisotropy ferromagnetic layer 12, with high-anisotropy ferromagnetic layer 12 between barrier layer 24 and the non-magnetic transition metal layer 14, and barrier layer 24 between high-anisotropy ferromagnetic layer 12 and reference layer 22. In some examples, barrier layer 24 includes MgO. In some examples, barrier layer 24 consists of or consists essentially of MgO.

Reference layer 22 may include any suitable material having a relatively fixed magnetization, for example, a fixed perpendicular magnetization. In some examples, reference layer 22 and composite layer 11 (for example, layers of composite layer 11) are both perpendicularly magnetized. An L1₀-FePt/MgO/L1₀-FePt MTJ and exchange-coupled CoFeB/MgO/CoFeB/Ru/CoFeB MTJ demonstrate TMR of 100%. In some examples, reference layer 22 may include L1₀-FePt with barrier layer 24 including MgO for sufficient readability. Read-out may be accomplished with a small read current flowing between reference layer 22 and another layer of composite free layer 11.

In some examples, article 10 may include respective interconnects between current or voltage sources and one or more layers of article 10, for example, layers of composite free layer 11 or other layers of article 11, for reading and writing. In some examples, article 10 may include at least one of read or write circuitry to cause a memory value (for example, a bit) to be written to or read from article 10. The memory value may be represented by a magnetization of composite free layer 11. For example, a first magnetic orientation of composite free layer 11 or of a layer of composite free layer 11 may denote a binary ‘1’, and a second magnetic orientation of composite free layer 1 or of a layer of composite free layer 11 may denote a binary ‘0’. In some examples, an example system, for example, a SOTRAM chip, may include an array of articles similar to article 10.

In some examples, an example spin-orbit-torque (SOT) stack may include article 10 or any example article according to the disclosure.

FIG. 2 is a flow diagram illustrating an example technique for forming an example article including a composite free layer. While the example technique of FIG. 2 is described with reference to article 10 of FIGS. 1A and 1B, example techniques according to the disclosure may be used to form any example articles according to the disclosure.

In some examples, the technique of FIG. 2 includes depositing non-magnetic transition metal layer 14 on ultra-low damping magnetic insulator 16 (32). Any suitable technique, such as chemical vapor deposition, physical vapor deposition, plasma deposition, or any suitable technique may be used for the depositing (32). In some examples, the depositing (32) of non-magnetic transition metal layer 14 may include sputtering a non-magnetic transition metal composition on ultra-low damping magnetic insulator 16. The non-magnetic transition metal composition may include a metal or alloy described with reference to non-magnetic transition metal layer 14 of FIG. 1A.

In some examples, the technique of FIG. 2 includes depositing high-anisotropy ferromagnetic layer 12 on non-magnetic transition metal layer (34). Any suitable technique, such as chemical vapor deposition, physical vapor deposition, plasma deposition, or any suitable technique may be used for the depositing (34). In some examples, the depositing (34) of high-anisotropy ferromagnetic layer 12 may include sputtering a high-anisotropy ferromagnetic composition on non-magnetic transition metal layer 14. The high-anisotropy ferromagnetic composition may include any suitable metal or alloy described with reference to high-anisotropy ferromagnetic layer 12 of FIG. 1A.

In some examples, the technique of FIG. 2 optionally includes, before the deposition (32) of non-magnetic transition metal layer 14, depositing ultra-low damping magnetic insulator 16 on conductive channel 18 an yttrium iron garnet (YIG) composition on conductive channel 18 (36) using pulse laser deposition. The depositing (36) may include growing layers of ferrimagnetic thulium iron garnet (TmIG) on gadolinium gallium garnet (GGG), for example, (111)-oriented GGG by pulse laser deposition.

FIG. 3 is a flow diagram illustrating an example technique for switching article 10 including composite free layer 11. While the example technique of FIG. 3 is described with reference to article 10 of FIG. 1, example techniques according to the disclosure may be used to switch any example articles according to the disclosure.

The example technique of FIG. 3 may include inducing spin orbit torque by passing a current through heavy metal region 20 of conductive channel 18 adjacent composite free layer 11 (42). The example technique may include switching a magnetization of ultra-low damping magnetic insulator 16 in response to the spin orbit torque (44). The example technique may include switching a perpendicular magnetization in high-anisotropy ferromagnetic layer 12 in response to the switching of the magnetization of ultra-low damping magnetic insulator 16 (46). The switching may be promoted by exchange coupling. For example, the switching may progress by domain wall switching, or a compound mechanism. In some examples, magnetic layers within high-anisotropy ferromagnetic layer 12 adjacent ultra-low damping magnetic insulator 16 may switch first, and promote the switching of an adjacent layer, ultimately resulting in successive switching of all layers high-anisotropy ferromagnetic layer 12 in a direction away from ultra-low damping magnetic insulator 16. Thus, ultimately, magnetization of high-anisotropy ferromagnetic layer 12 may be switched by changing a current or reversing a current in conductive channel 18.

Article 10 may be switched when a critical charge current density (J_(c)) passes through heavy metal region 20, inducing spin accumulation at the HM/FM interface (for example, an interface between composite free layer 11 and heavy metal region 20) due to spin-orbit interaction via a spin-Hall effect. Spin accumulation density (J_(s)) is given by EQUATION 1.

$\begin{matrix} {J_{s} = {{\Phi_{H}\left( {1 - {{sech}\left( \frac{t_{H}}{\lambda_{H}} \right)}} \right)}\frac{J_{c}}{e}}} & \left( {{Equation}\mspace{14mu} 1} \right) \end{matrix}$

where Φ_(H), t_(H), λ_(H), and e are the spin hall angle. HM thickness (“HM” denotes heavy metal region 20). HM spin-flip scattering length, and electron charge, respectively. The spin hall angle describes the maximum efficiency of the HM to convert charge current density into spin accumulation density. The thickness dependence of this conversion captures the spin backscattering from the bottom surface of the HM.

A spin-orbit torque is generated in the transverse direction (denoted by 6 in FIG. 1A) and reverses the magnetization of the easily-switched ultra-low damping magnetic insulator 16. This generates an exchange field from ultra-low damping magnetic insulator 16 that switches high-anisotropy ferromagnetic layer 12. In some examples, the anisotropy axes of ultra-low damping magnetic insulator 16 and high-anisotropy ferromagnetic layer 12 point in the {circumflex over (σ)} direction, which may attenuate the spin backflow, for example, via a FeMn spin sink underlayer.

For a desired thermal stability (A), the write energy density may be given by EQUATION 2:

$\begin{matrix} {E_{W} = {{t_{p}R_{H}I_{c}^{2}} = {t_{p}\rho_{H}{{ldt}_{H}\left( \frac{{e}J_{s}}{\Phi_{H}\left( {1 - {{sech}\left( \frac{t_{H}}{\lambda_{H}} \right)}} \right)} \right)}^{2}}}} & \left( {{Equation}\mspace{14mu} 2} \right) \end{matrix}$

where t_(p), I_(c), R_(H), ρ_(H), l, and d are the current pulse duration, critical charge current, HM electrical resistance, HM resistivity, and HM length, respectively. The generalized preferable HM thickness (t_(H,opt)) is obtained via the first and second derivative tests: t_(H,opt)=2.45λ_(H). EQUATION 2 transforms to EQUATION 3:

$\begin{matrix} {E_{W} \cong {3.57t_{p}\rho_{H}d^{2}{\lambda_{H}\left( \frac{{e}J_{s}}{\Phi_{H}} \right)}^{2}}} & \left( {{Equation}\mspace{14mu} 3} \right) \end{matrix}$

where l≅d (similar length and diameter). From EQUATION 3, a figure of merit (FOM) for HM layer selection is given by EQUATION 4:

$\begin{matrix} {{FOM}_{H} = \frac{\Phi_{H}^{2}}{\rho_{H}\lambda_{H}}} & \left( {{Equation}\mspace{14mu} 4} \right) \end{matrix}$

where the best heavy metals have the largest FOM_(H). A large spin hall angle results in smaller charge current density. Small resistivity reduces bias voltage. Small spin-flip scattering length reduces thickness. This linearly increases resistance but decreases charge current, which affects E_(W) quadratically. This figure of merit may be used to select suitable HM materials for heavy metal region 20.

The desired Δ is calculated from the sum of the anisotropy energy, the demagnetization energy, and exchange energy of the entire structure. Magnetostatic interaction is neglected owing to the stray-field free nature of this cell and the large exchange coupling between, for example, ultra-low damping magnetic insulator 16 (for example, YIG) and high-anisotropy ferromagnetic layer 12 (for example) FePt. A may be obtained by EQUATION 5.

Δ=¼πd ²(t _(ULD) K _(ULD) +t _(FM) K _(FM))≅¼πd ² t _(FM) K _(FM)  (Equation 5)

where t_(ULD) and t_(FM) are the thicknesses of ultra-low damping magnetic insulator 16 and high-anisotropy ferromagnetic layer 12, respectively. This agrees with previous Δ calculations for composite structures. The ULD contribution is negligible, because K_(ULD)≅10⁻⁴K_(FM). A thermal stability Δ=60 k_(B)T ensures a data retention of 10 years. EQUATION 3 may be rewritten as EQUATION 6:

$\begin{matrix} {E_{W} \cong {\frac{272.55\mspace{11mu} t_{p}k_{B}T\; \rho_{H}\lambda_{H}}{t_{F\; M}K_{F\; M}}\left( \frac{{e}J_{s}}{\Phi_{H}} \right)^{2}}} & \left( {{Equation}\mspace{14mu} 6} \right) \end{matrix}$

The critical spin accumulation density (J_(s)) is calculated via the 4^(th) Order Runge-Kutta numerical integration of the set of coupled Landau-Lifshitz equations with the SOT term described in EQUATIONS 7 and 8:

$\begin{matrix} {\frac{d\; {\hat{m}}_{ULD}}{dt} - {\gamma \; {\hat{m}}_{ULD} \times {\overset{\rightarrow}{H}}_{ULD}} - {\gamma \; \alpha_{ULD}{\hat{m}}_{ULD} \times \left( {{\hat{m}}_{ULD} \times {\overset{\rightarrow}{H}}_{ULD}} \right)} + {\overset{\rightarrow}{\tau}}_{she}} & \left( {{Equation}\mspace{14mu} 7} \right) \\ {\frac{d\; {\hat{m}}_{F\; M}}{dt} = {{{- \gamma}{\hat{\; m}}_{F\; M} \times {\overset{\rightarrow}{H}}_{F\; M}} - {\gamma \; \alpha_{F\; M}{\hat{m}}_{F\; M} \times \left( {{\hat{m}}_{F\; M} \times {\overset{\rightarrow}{H}}_{F\; M}} \right)}}} & \left( {{Equation}\mspace{14mu} 8} \right) \end{matrix}$

where {circumflex over (m)}_(ULD), {right arrow over (H)}_(FM), {circumflex over (m)}_(FM), {right arrow over (H)}_(FM), γ, and {right arrow over (τ)}_(she) are the unit magnetization vector of ultra-low damping magnetic insulator 16, the effective field on ultra-low damping magnetic insulator 16, unit magnetization vector of high-anisotropy ferromagnetic layer 12, effective field on high-anisotropy ferromagnetic layer 12, gyromagnetic ratio, and the SHE torque, respectively. The damping term of EQUATION 7 is negligible since α_(ULD)≅10⁻⁵. The effective field imposed on ultra-low damping magnetic insulator 16 is approximately equal to the exchange field imposed on it by high-anisotropy ferromagnetic layer 12, since its coercivity and saturation magnetization (M_(s,ULD)=75 emu/cc) are relatively very small, as given by EQUATION 9:

$\begin{matrix} {{\overset{\rightarrow}{H}}_{ULD} \cong {\frac{J_{ex}}{t_{ULD}M_{s,{ULD}}}{\hat{m}}_{ULD}}} & \left( {{Equation}\mspace{14mu} 9} \right) \end{matrix}$

The effective field of high-anisotropy ferromagnetic layer 12 is expressed by EQUATION 10:

$\begin{matrix} {{\overset{\rightarrow}{H}}_{F\; M} = {{\frac{J_{ex}}{t_{F\; M}M_{s,{F\; M}}}{\hat{m}}_{ULD}} - {{\overset{\leftrightarrow}{N}}_{d\; m}M_{s,{F\; M}}{\hat{m}}_{F\; M}} + {\frac{2K_{F\; M}}{M_{s,{F\; M}}}{\hat{m}}_{{F\; M},z}}}} & \left( {{Equation}\mspace{14mu} 10} \right) \end{matrix}$

where M_(s,FM),

_(dm), and {circumflex over (m)}_(FM,z), are the saturation magnetization of high-anisotropy ferromagnetic layer 12 (1140 emu/cc for FePt), demagnetization tensor for high-anisotropy ferromagnetic layer 12, and z-component of the unit magnetization vector for high-anisotropy ferromagnetic layer 12, respectively.

The SOT is given by EQUATION 11:

$\begin{matrix} {{\overset{\rightarrow}{\tau}}_{she} = {{- \gamma}{\frac{\hslash \; J_{s}}{2M_{s,{ULD}}t_{ULD}}\left\lbrack {{{\hat{m}}_{ULD} \times \left( {\hat{\sigma} \times {\hat{m}}_{ULD}} \right)} + {r_{\bot{/ \parallel}}\left( {\hat{\sigma} \times {\hat{m}}_{ULD}} \right)}} \right\rbrack}}} & \left( {{Equation}\mspace{14mu} 11} \right) \end{matrix}$

where ℏ is the reduced Planck constant and r_(⊥/∥) is the ratio of the out-of-plane field-like torque to the in-plane Slonczewski-like torque. This value is approximately 1.0 for a HM layer (for example, heavy metal region 20) with t_(H)>λ_(H) and adjacent FM layer (e.g., YIG) with t_(ULD)≅λ_(ϕULD), where λ_(ϕULD) is the spin dephasing length of ultra-low damping magnetic insulator 16 (λ_(ϕULD)≅30 nm). The field-like term is typically neglected in spin-valves with dephasing lengths much smaller than their thickness, but the dephasing length of ultra-low damping magnetic insulator 16 may be on the order of its thickness in some examples.

Thus, in some examples, the write field for composite free layer 11 may scale proportionally to a thickness of high-anisotropy ferromagnetic layer 12 (t_(FM)). Similarly, an increase in t_(FM) yields a proportional J_(s). Therefore, J_(s)∝t_(FM)∝d⁻² and E_(w)∝d⁻². The scaling may be a function of the thicknesses of ultra-low damping magnetic insulator 16, non-magnetic transition metal layer 14, and high-anisotropy ferromagnetic layer 12 (t_(Rh) may determines J_(ex)).

EXAMPLES Example 1

Composite structures having diameters of 13.2 nm, 9.4 nm, and 6.6 nm were simulated. The initial angle was estimated from thermal stability by: θ₀=√{square root over (1/Δ)}≅0.13 rad. Similarly, the critical angle for magnetization switching is θ₀=π−θ₀ and is defined as the angular distance the magnetization must travel to constitute a thermally stable reversal. Both free layers must travel θ_(c) within 1 ns for ultrafast information storage. The numerical integration time step is 10 fs. The spin-hall angle is set to 0.3—achievable with β-W (β-tungsten) thin films, which have ρ_(H)=200 μΩcm and λ_(H)=1.4 nm. Therefore, FOM_(H) of β-W is 31.1 kΩnm².

FIG. 4A is a chart illustrating relationships between write energy, exchange coupling, and magnetic insulator thickness for an example device having a diameter of 13.2 nm. FIG. 4B is a chart illustrating relationships between write energy, exchange coupling, and magnetic insulator thickness for an example device having a diameter of 9.4 nm. FIG. 4C is a chart illustrating relationships between write energy, exchange coupling, and magnetic insulator thickness for an example device having a diameter of 6.6 nm. The contours of FIGS. 4A, 4B, and 4C illustrate the E_(w) scaling for structures with a diameter of 13.2 nm, 9.4 nm, and 6.6 nm. These diameters correspond to FePt thicknesses of 0.25 nm, 0.5 nm, and 1 nm, respectively. Thicknesses below 4 Å may not be practical for FePt, but the central argument may hold for other high K_(u) materials such as FePd.

Example 2

The effect of changing the thickness of YIG (the ULD included YIG) and the exchange coupling were evaluated. A wide range of t_(ULD) and J_(ex) were explored for each device size—i.e., 1.9-32 nm and 0-34 erg/cm². A preferable point was obtained at

$\frac{t_{ULD}M_{s,{ULD}}}{t_{F\; M}M_{s,{F\; M}}} = {{2\mspace{14mu} {and}\mspace{14mu} \frac{J_{ex}}{2t_{F\; M}K_{,{F\; M}}}} = 0.57}$

for each device size—i.e., the preferable J_(ex) and t_(γIG) scale proportionately with device size.

FIG. 5A is a chart illustrating relationships between write energy and diameters of example and comparative memory structures. FIG. 5B is a chart illustrating relationships between write energy and diameters of example and comparative memory structures. The E_(w) of a single 60 kBT L1₀-FePt layer is calculated and included in FIG. 5A for comparison. At a diameter of 13.2 nm, the composite structure achieves E_(w)=10 aJ, while the single layer structure obtains E_(w)=182 aJ. For d=3.3 nm, the composite structure achieves E_(w)=138 aJ, while the single layer structure obtains E_(w)=46×10³ aJ. Therefore, the improvement ranges from 18-337×, suggesting that the composite free layer structure is the superior choice for ultra-high-density memory.

FIG. 6A is a chart illustrating magnetization reversal of an ultra-low damping magnetic insulator in an example antiferromagnetically coupled composite structure. FIG. 6B is a chart illustrating magnetization reversal of a high-anisotropy ferromagnetic layer in the antiferromagnetically coupled composite structure of FIG. 6A. FIG. 7 is a chart illustrating magnetization switching of an ultra-low damping magnetic insulator and a high-anisotropy ferromagnetic layer in an example antiferromagnetically coupled composite structure. FIGS. 6A and 6B illustrate the magnetization reversal of both layers for an example structure with d=13.2 nm. The YIG layer experienced a SOT at 0 ns and began to switch, exerting its exchange field onto the FePt. At roughly 0.8 ns, this field was sufficient to switch the FePt magnetization. The comparative STT-MTJ can achieve an E_(w) of approximately 10⁵ aJ, which is 10⁴× more write energy than the example composite cell (FIG. 5A). Moreover, these devices are only 40 k_(B)T with d=40 nm. A larger requisite device diameter results in larger write energy. Therefore, employment of high K_(u) materials such as L1₀-FePt may be advantageous in SOT-RAM.

The state-of-the-art DDR4 DRAM has a standard cell area of 6 F², where F denotes the process—e.g., the state-of-the-art process is 10 nm. It operates at 1V with a cell capacitance of approximately 10 fF. Therefore, the well-known write energy of a DRAM cell is 10⁴ aJ (FIG. 5A)—a 10³ increase in E_(w) compared to the composite structure. As seen in FIG. 5B, for a FePd/YIG composite free layer, E_(w) could be reduced to 18 aJ (4300 k_(B)T) for Ins switching, a factor of 500× less than DDR4-DRAM (10 fL).

By utilizing the exchange coupling between thermally stable L10-FePt and an ultra-low damped magnetic insulator such as YIG, a write energy of 10 aJ was achieved.

Example 3

The effect of varying exchanging coupling, thickness of magnetic insulating layer, and thickness of high-anisotropy layer was evaluated by simulation. Δ was maintained at 60. FIG. 8A is a chart illustrating relationships between write energy, exchange coupling, and magnetic insulator thickness for an example device having a diameter of 8.3 nm and including an FePt high-anisotropy ferromagnetic layer. The thickness of the FePt layer was 1 nm, and E_(w,min) was about 137 aJ.

FIG. 8B is a chart illustrating relationships between write energy, exchange coupling, and magnetic insulator thickness for an example device having a diameter of 15.6 nm and including an FePd high-anisotropy ferromagnetic layer. The thickness of the FePt layer was 1 nm, and E_(w,min) was about 60 aJ.

Example 4

Bit error rates were compared by micromagnetic and macrospin simulations. FIG. 9 is a chart illustrating bit error rates for macrospin and micromagnetic simulations for an example composite structure including a Ytrrium iron garnet (YIG) ultra-low damping magnetic insulator and an FePd high-anisotropy ferromagnetic layer. The FePd layer had a 1 nm thickness, and the YIG layer had a thickness of 2.14 nm. The J_(ex) was 0.5 ergs/cm², with diameter of 15.6 nm. The cell size for micromagnetic simulation was 2.6 nm×2.6 nm×1 nm. Micromagnetic simulation showed switching at a lower current than macrospin simulations.

Various examples have been described. These and other examples are within the scope of the following claims. 

1. An article comprising: a composite free layer, the composite free layer including: a high-anisotropy ferromagnetic layer, a non-magnetic transition metal layer adjacent to the high-anisotropy ferromagnetic layer, and an ultra-low damping magnetic insulator, wherein the non-magnetic transition metal layer is between the high-anisotropy ferromagnetic layer and the ultra-low damping magnetic insulator; and a conductive channel including a heavy metal region adjacent the composite free layer, wherein the ultra-low damping magnetic insulator is between the non-magnetic transition metal layer and the conductive channel.
 2. The article of claim 1, wherein the high-anisotropy ferromagnetic layer comprises L1₀ FePt or L1₀ FePd.
 3. The article of claim 1, wherein the high-anisotropy ferromagnetic layer has an anisotropy in a range from about 1×10⁶ ergs/cc to about 4×10⁷ ergs/cc.
 4. The article of claim 1, wherein the high-anisotropy ferromagnetic layer has a thickness in a range from about 3 Angstroms (Å) to about 5 nanometers (nm).
 5. The article of claim 1, wherein the non-magnetic transition metal layer has a thickness of less than about 15 Angstroms (Å).
 6. The article of claim 1, wherein the non-magnetic transition metal layer comprises rhodium.
 7. The article of claim 1, wherein the ultra-low damping magnetic insulator comprises an yttrium iron garnet (YIG).
 8. The article of claim 1, wherein the heavy metal region of the conductive channel comprises β-tungsten.
 9. The article of claim 1, further comprising a reference layer and a barrier layer adjacent the high-anisotropy ferromagnetic layer, wherein the barrier layer is between the reference layer and the high-anisotropy ferromagnetic layer, and wherein the high-anisotropy ferromagnetic layer is between the barrier layer and the non-magnetic transition metal layer.
 10. The article of claim 9, wherein the barrier layer comprises MgO.
 11. A memory device comprising: a composite free layer, the composite free layer including: a high-anisotropy ferromagnetic layer, a non-magnetic transition metal layer adjacent to the high-anisotropy ferromagnetic layer, and an ultra-low damping magnetic insulator, wherein the ultra-low damping magnetic insulator comprises an yttrium iron garnet (YIG) or barium ferrite, and wherein the non-magnetic transition metal layer is between the high-anisotropy ferromagnetic layer and the ultra-low damping magnetic insulator; and a conductive channel including a heavy metal region adjacent the composite free layer, wherein the ultra-low damping magnetic insulator is between the non-magnetic transition metal layer and the conductive channel.
 12. The memory device of claim 11, further comprising a reference layer and a barrier layer adjacent the high-anisotropy ferromagnetic layer, wherein the barrier layer is between the reference layer and the high-anisotropy ferromagnetic layer, and wherein the high-anisotropy ferromagnetic layer is between the barrier layer and the non-magnetic transition metal layer.
 13. The memory device of claim 11, wherein the high-anisotropy ferromagnetic layer comprises L1₀ FePt or L1₀ FePd.
 14. The memory device of claim 11, wherein the non-magnetic transition metal layer has a thickness of less than about 15 Angstroms (Å), and wherein the non-magnetic transition metal layer comprises rhodium.
 15. The memory device of claim 11, wherein the memory device is a random access memory (RAM) device.
 16. A method comprising: depositing a non-magnetic transition metal layer on an ultra-low damping magnetic insulator, wherein the ultra-low damping magnetic insulator is on a conductive channel comprising a heavy metal region; and depositing a high-anisotropy ferromagnetic layer on the non-magnetic transition metal layer.
 17. The method of claim 16, wherein depositing the non-magnetic transition metal layer comprises sputtering a non-magnetic transition metal composition on the ultra-low damping magnetic insulator.
 18. The method of claim 16, wherein depositing the high-anisotropy ferromagnetic layer comprises sputtering a high-anisotropy ferromagnetic composition on the non-magnetic transition metal layer.
 19. The method of claim 16, further comprising depositing the ultra-low damping magnetic insulator on the conductive channel by pulse laser depositing an yttrium iron garnet (YIG) composition on the conductive channel. 